Liquid level gauge



my w50 s. J. SMITH 2,534,802

LIQUIDfLEVEL GAUGE Filed April 8. 1946 Inventor b Stanley @mes 'rm't/l.

y -wwm7 CMQG/Mfeg ttorneys' Patented Dec. 19, 1950 LIQUID LEVEL GAUGE Stanley James Smith, London, England, assignor to West Road Co. Limited, a British company Application April 8, 1946, Serial No. 660,583 In Great Britain April 9, 1945 2 Claims. 1

This invention relates to liquid level gauges of the kind in which the rise and fall of the liquid varies the electrical capacity of a condenser. This condenser may comprise two spaced conductive electrodes arranged vertically in the liquid container or, in the case where the liquid is itself conductive a single insulated electrode may be used, the liquid forming the other electrode. In such arrangements the' liquid level is measured by measuring the capacity of the condenser and for this purpose a source of alternating current is required. In the arrangement described in my prior United States Patent No. 2,377,275 the primary source of power was a low voltage battery and alternating current of suitable frequency was generated by means of a vacuum tube oscillator.

In some applications the use of batteries and vacuum tubes is inconvenient and a principal object of the present invention is to provide an alternative circuit arrangement which may be operated from a sourcel of alternating current.

A further object of the present invention is to provide a novel arrangement for measuring or indicating the capacity of a condenser.

One form of the present invention will now be described with reference to the accompanying drawing in which is shown a circuit arrangement for measuring or indicating the capacity of a condenser.

A source of alternating current I is connected with the primary winding 2 of a transformer 3. This transformer is designed to have an iron core which becomes saturated under working conditions so that the output from the secondary winding 4 has a considerably distorted wave form and thus contains many harmonics. The output is fed over a cable 5 to a circuit, including inductance 6 and condenser l, which is tuned to the frequency of a desired harmonic of the source. The output of the circuit 6, I is applied to a bridge network 8 which is made up of the condenser 9, the condenser I0, and the primary winding II of transformer I2. The condenser 9 is normally fixed but may be adjustable for zero setting or like purposes. The condenser IU is the condenser whose value is to be measured or indicated, for example the measuring condenser of a liquid contents gauge. The condenser I is preferably connected with the bridge 8 over a low capacity cable I8 such as a co-axial cable. The secondary winding I3 of the transformer I2 is connected with the moving coil of an indicating instrument I through a rectifier I6, over the cable I'I.

lf the condensers 9 and IU are of equal value the bridge network will be balanced and no current will ow through the indicator I5. If, however, the condensers are unequal current will ow through the indicator and if the value of the condenser 9 is maintained constant the scale of the indicator may be calibrated to indicate the magni-4 tude of the condenser I0. Thus if this condenser is the measuring condenser of a liquid level indicating system the scale of the indicator I5 may be calibrated in terms of liquid level.

The frequency of operation of the bridge circuit will be governed to some extent by the size of the condenser I 0. Thus in one application for a liquid level gauge on aircraft where a source of 400 c. p. s. alternating current is available a suitable frequency of the supply to the bridge circuit 8 was 2 k. c. s. and the circuit 6, 'I was accordingly tuned to select the fth harmonic of the frequency of the source I.

Errors in the reading of the indicator I5 due to variations in the supply voltage, which normally preclude the use of a bridge circuit operating in an unbalanced condition, are practically eliminated by the self-regulating action of the saturated transformer 3. This function of a saturated reactor is well known to those skilled in the art and it is sucient to say that the output of the transformer 3 when saturated, at fundamental or at harmonic frequency, can be made to be substantially independent of the input. In addition compensation for fluctuations in frequency of the source I can be provided by tuning the circuit 6, I to a frequency somewhat below the frequency of the desired harmonic so that if the frequency of the source increases the output to the bridge will be reduced thus off-setting the reduced reactance of the condenser I0, and vice versa.

The compensation thus provided enables the very simple measuring circuit described to be used and the whole arrangement, with the exception of the measuring condenser Ill and the indicator I5, can be constructed as a self-contained unit of very small volume. In practice this unit may be connected with the source I and transformer 3, and with the indicator I5, over cables of any desired length while the connection between the unit and the measuring condenser IU may also be of some length if the capacity of the condenser I0 is not too low and cable of small capacity is used.

I claim:

1. An electrical circuit arrangement for measuring capacity comprising a bridge network including in series a relatively xed condenser, the condenser to be measured and the primary winding of a transformer, an indicating instrument connected with a secondary winding on said transnectingsaidcircuitaorolethebridgenetwork former and means for supplying alternating eurrent to said network comprising a saturable reactor for connection with a source of alternating current, a circuit fed from said reactor and tuned toaharmonic ofsaidsource, andmeansconneotingsaidcircuitacrossoneofsaidcondenseraand partoftheprimarywindingofsaidtranafm'mer for feeding the harmonic frequency outmt of said circuit to said bridge network.

2. An electrical circuit arrangement for measuring capacity comprising a bridge network inm cluding in series four arms of which two consist of a relatively nxed condenser and the condenser to be measured and the other two -arms consist of the center tapped primary winding of a transformer, an indicating instrument connected with a secondary winding on the said transformer, and means for supplying alternating current to said network comprising a saturable transformer having a primary winding for connection with a source of alternating current and a secondary winding, a circuit tuned to a harmonic of said source and connected with the secondary winding -of said saturable transformer, and means conforsuwlylngtheretoeurrentatthenidharmonie STANLEY JAKE MTH.

:masons cr'rm The following references are of record in the file of this patent:

UNITED STATES PATENTE 

